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The H1N1 vaccine will arrive too late to help most Americans who will be infected during this flu season, according to a study conducted by scholars at Purdue University.

The study also estimates that the virus - commonly referred to as the swine flu bug - will infect about 60 percent of the U.S. population, although only about 25 percent of Americans will fall ill.

Published Oct. 15 in Eurosurveillance, a scientific journal devoted to epidemiology and the surveillance and control of communicable diseases, the study was conducted by professors Sherry Towers and Zhilan Feng of, respectively, Purdue’s statistics and mathematics departments.

“The model predicts that there will be a significant wave in autumn, with 63% of the population being infected, and that this wave will peak so early that the planned [U.S. Centers for Disease Control and Prevention] vaccination campaign will likely not have a large effect on the total number of people ultimately infected by the pandemic H1N1 influenza virus,” the authors wrote in their study.

The authors said that this is the week, through Oct. 24, during which the greatest number of people would be infected. The vaccination program has barely started in the U.S.

“The model predicts that the peak wave of infection will occur near the end of October in week 42, with 8% of the population being infected during that week. By the end of 2009, the model predicts that a total of 63% of the population will have been infected,” the authors wrote in a conclusion that ignored the effects of a CDC vaccination program.

But even “when the model was modified to include the effect of the planned vaccination scheme,” that adjustment yielded only “a relative reduction of about 6% in the total number of people infected” by the end of the year 2009.

Ms. Towers cautioned in a phone interview with The Washington Times that while enough of the U.S. population probably won’t get enough of the vaccine before or during the peak of the pandemic, that is no reason not to get protection.

“Based on our study alone it would be bad to discourage people from getting the vaccine, because what if our study is wrong,” she said.

She called some comments in the press “a lot of hysteria. In reality, the overwhelming majority of people getting sick are going to have a mild illness.”

The study cautioned that while a majority of the U.S. population would become infected by the virus, previous studies “showed that up to 60% of seasonal influenza infections are asymptomatic.” That is, people became infected but showed no symptoms.

“If the same is true of the current pandemic influenza, about a quarter of the population will fall ill,” the authors wrote.

Canada is in a much more dire straits on vaccination, Ms. Towers noted in her interview, because of relative slowness of the country’s equivalent of the Food and Drug Administration (FDA) to approve the vaccine.

“They are more cautious and conservative and sitting on at least 1 million doses waiting for delivery. They won’t begin vaccination until the first week of November,” she said.

However, the study, which used data only though the week ending Aug. 22, assumed that “the planned CDC vaccination program against pandemic H1N1 influenza will begin with six to seven million doses being delivered by the end of the first full week in October.”

That assumption turned out to be more optimistic about the program’s numbers than turned out to be case - about 6.8 million doses were available and 3.7 million doses had been ordered, the CDC said on Oct. 9.

Shipments reported Friday were down by a quarter, because of a slowdown in production caused by difficulties of processing great numbers.

The study also assumed that all adults would achieve full immunity shortly after receiving inoculations and assumed that children and adults are equally responsive to the flu vaccine. Both assumptions are medically dubious, though hard to compensate for statistically.

All these statistical assumptions and prediction errors would lead the study to underestimate the number of infections.

The study was based on a statistical analysis of weekly reports from the CDC for a model derived from the observed spread of the H1N1 virus “during summer 2009 to predict the behavior of the pandemic during autumn 2009.”

A White House scenario released in late August by a group of presidential advisers had predicted that anywhere from 30 percent to 50 percent of the population would get the novel virus strain, and an estimate on the number of deaths ranged from 30,000 to 90,000. The regular seasonal flu normally affects up to 20 percent of the population, with 36,000 deaths resulting.

Dr. Thomas Frieden, CDC director, cautioned at the time that such a prognosis was unlikely unless the virus changed and mutated into another form, which, to date, has not happened. Further estimates and availability of vaccine doses are due from CDC officials Tuesday.

But the Purdue scholars’ study predicts a higher rate of infection and illness than previous projections because, the authors explained to The Washington Times in separate e-mails, they used a better formula - “a seasonally forced SIR model” that adjusts the rates of infection spread for the different times of the year.

“However, one aspect of influenza that needs to be added to the [SIR] model is its seasonal nature; it is more infectious in the fall and winter in the northern hemisphere than it is in the summer. We refer to this as ‘seasonal forcing.’ … Our model takes this seasonal forcing into account,” Ms. Towers wrote.